The operation of vapor compression systems with CO2 as the refrigerant is characterized by the low critical temperature of CO2 at approximately 31° C. At many operating conditions, the critical temperature of CO2 is lower than the temperature of the heat sink, which results in a transcritical operation of the vapor compression system. In the transcritical operation the heat rejection occurs at a pressure above the critical pressure, and the heat absorption occurs at a pressure below the critical pressure. The most significant consequence of this operating mode is that pressure and temperature during the heat rejection process are not coupled by a phase change process. This is distinctly different from conventional vapor compression systems, where the condensing pressure is linked to the condensing temperature, which is determined by the temperature of the heat sink. In transcritical vapor compression systems, the refrigerant pressure during heat rejection can be freely chosen, independent of the temperature of the heat sink. However, given a set of boundary conditions (temperatures of heat sink and source, compressor performance, heat exchanger size, and line pressure drops) there is a first “optimum” heat rejection pressure, at which the energy efficiency of the system reaches its maximum value for this set of boundary conditions. There is also a second “optimum” heat rejection pressure, at which the cooling capacity of the system reaches its maximum value for this set of boundary conditions. The existence of these optimum pressures has been documented in the open literature. For example, maximum energy efficiency is attained in U.S. Pat. Nos. 6,568,199 and 7,000,413, and maximum heating capacity is attained in U.S. Pat. No. 7,051,542, all of which are assigned to the assignee of the present invention.
Given a set of boundary conditions (temperature of heat source, compressor performance, heat exchanger size, and line pressure drops), the value of the optimum heat rejection pressure depends primarily on the temperature of the heat sink. Conventional control schemes for CO2 systems utilize the refrigerant temperature at the heat rejection heat exchanger outlet or the heat sink temperature or any indicator of these as the control input to control the heat rejection pressure. However, in systems designed for an operating envelope which covers a large range of heat source temperatures (e.g. −20 F to 57 F), such as transport refrigeration units, it may not be sufficient to correlate the optimum high-side pressure only to the temperature of the heat sink.